1. Field of the Invention
The present invention generally relates to a drive system for an off-road vehicle, and more particularly to an improved drive system suitable for an off-road vehicle.
2. Description of Related Art
Off-road vehicles are designed to be operated over rugged terrain. These vehicles are often operated off paved roads in areas featuring terrain such as, for example, grassy meadows, steep inclines and hills, rough stone and/or dirt roads, mud holes, creeks and shallow ponds.
The off-road vehicles can include a frame supported by wheels. In some arrangements, the vehicle has a pair of front wheels and a pair of rear wheels. An internal combustion engine drives at least the front pair of wheels or the rear pair of wheels. In some arrangements, the engine can drive all four wheels. The engine drives the wheels through a drive system and a drive system that transmits the engine power to all of the wheels is commonly called a four-wheel drive system. Typically, the four-wheel drive system includes a front differential mechanism that is associated with the front wheels and a rear differential mechanism that is associated with the rear wheels. Japanese Patent Publication No. 2000-103246 discloses a vehicle that has an exemplary rear differential mechanism.
The differential mechanisms normally include a gear train within a housing. The gear train is coupled with half shafts that extend to the respective wheels. In other words, a pair of half shafts are driven by power transmitted through each differential gear train. Differential mechanisms allow the two associated wheels to turn at different speeds. For instance, when the vehicle turns, the outside wheel of the turn (e.g., the left wheel when turning right) spins faster than the inside wheel. This is particularly pronounced at the rear wheels.
In some arrangements, the vehicles can feature a center differential mechanism that couples a front driveshaft that extends to the front differential with a rear driveshaft that extends to the rear wheels. The center differential allows the front wheels to turn at a different speed than the rear wheels. Thus, the center differential, in effect, absorbs a difference in rotational speed among the respective four wheels when it works in cooperation with the front and rear differential mechanisms. Such differential mechanisms are well known to those of skill in the art.
As a result of the desirable operational characteristics of the differential mechanisms, the wheel having the greater traction of the two receives less power from the engine relative to the wheel having less traction. Thus, if one wheel loses traction in mud, snow or the like, then the wheel that has lost traction will receive more power from the engine. In an extreme situation, all of the engine power may be transmitted to a wheel that simply spins relative to the ground and cannot gain traction. The wheel that has relatively more traction, therefore, effectively has power robbed from it in favor the wheel with relatively less traction. This can result in the vehicle becoming stuck in the mud, snow or the like.
Accordingly, locking differentials are provided that allow an operator to lock the differential such that both associated wheels are forced to rotate at the same speed. Such mechanisms keep a wheel that is losing traction from depriving the other wheel of engine power. In such arrangements, however, the center differential still may cause the wheels getting the best traction to receive less power than the wheels getting the worst traction. In other words, the front wheels could be slipping, which will cause the center differential to direct more engine power to the front wheels. Thus, the center differential can decrease the ability of the vehicle to escape from a low traction environment, such as mud, snow, climbing or descending hills and rocks or the like.